After being there a few years, a human by the name of Kasper has settled down. He's got his own place on the outskirts of Ponyville, and makes a fine living working at Sugarcube Corner. This also means that Pinkie and himself are very close. One day, Pinkie informs Kasper that her sister, Maud, was going to be moving in with her.
That's when things take a turn for the worse…

(I wasn't quite able to track down who the artist of the cover art is from, so if anyone could help with that, i'd be grateful).